This invention relates to the selection and use of certain pressure-sensitive adhesive-coated thermoplastic label materials for identification and marking of cryogenic vials and other containers subjected to very low temperatures More specifically, the invention relates to, and derives from the discovery that very ductile vinyl (PVC) labels can resist adhesive delamination and label edge peeling at cryogenic temperatures particularly if the PVC is plasticized with a non-mobile plasticizer, and the labels are oriented so that their more stretchable direction is oriented around the circumference of a cryogenic container.
Storage of perishable or unstable biological, chemical and industrial materials including tissue culture cells, tissues, embryos, sperm cells, eggs, chemicals, biochemicals and the like, at low and ultra-low temperatures, is referred to as cryogenic storage. For the purposes of this invention, cryogenic storage temperatures are defined as temperatures ranging downward from approximately −80 degrees C. to at least −196 degrees C. (the boiling point of liquid nitrogen), and are provided by special mechanical freezers and by special insulated Dewar chambers carrying liquid nitrogen. Temperatures as low as −270 degrees C. are provided by Dewar chambers holding liquid helium.
Tightly sealing cylindrical vials having gasketed sealing lids, are typically fabricated from thermoplastic materials such as polypropylene and polyethylene, and are commercially available for holding samples under cryogenic storage conditions. These vials are often essential for sample storage in liquid nitrogen at −196 degrees C., or at the same temperature in the vapor phase above liquid nitrogen. Microcentrifuge tubes (typically holding between 0.5 ml–2.5 ml liquid) formed from similar plastics, and having rounded or conical bottoms and tightly sealing screw cap or frictionally sealing hinged snap cap closures, are also used for cryogenic storage of samples. While it is possible to manually write on such round-surfaced vials and tubes and other cryogenic storage containers with permanent marking ink for purposes of sample identification, it is considerably easier to place a pre-marked or pre-printed label on such a container.
U.S. Pat. No. 5,836,618 by Perlman, describes non-vinyl pressure-sensitive container labels capable of withstanding cryogenic storage conditions. Perlman also describes prior art label materials for cryogenic containers, including woven and non-woven fiber and cloth tapes, and a hand-markable vinyl label that could not retain computer-directed printing (such as ink-jet or laser printing). That prior art vinyl label (corresponding to product number 7604FP manufactured by the 3M Company, St. Paul, Minn., and previously advertised and sold by Diversified Biotech, Boston, Mass.) had limited utility because it was not machine-printable and was also susceptible to partial peeling on low surface energy, curved surface plastic vials such as polypropylene microcentrifuge tubes that were exposed to liquid nitrogen. Peeling of 7604FP labels was more problematic with age, and may have resulted from the combination of vinyl plasticizer and adhesive used in this product. Examples of the other previously available cryo-label materials include cloth Cryoware™ labels from the Nalge Company (Rochester, N.Y.) and “high/low temperature” cloth tape #314 from TimeMed Labeling Systems, Inc. (Burr Ridge, Ill.). Also, “Clear Tape” that is produced by Bel-Art Products (Pequannock, N.J.) is not actually a label, but is a clear overcoating tape to protect labels, and can withstand liquid nitrogen exposure.
U.S. Pat. No. 5,836,618 by Perlman is relevant background to the present invention, and is incorporated herein by reference in its entirety. Perlman describes a number of non-polyvinyl, i.e., non-PVC-based stretchable cryogenic labels in which the facestock materials could be stretched, i.e., elongated prior to breakage, at least 10% in both the machine direction (abbreviated MD) and the transverse direction (abbreviated TD) of the facestock without breaking. Standard methods for testing labels for such percentages of elongation prior to breakage, e.g., ASTM D-882A, are described in American Standard Testing Method (ASTM) manuals.
In contrast to the ASTM test, a simplified method was utilized for testing the extent of stretch during the period of initial research for U.S. Pat. No. 5,836,618, employing narrow strips of label material (approximately ⅛ inch in width and 6–8 inches in length cut in either the MD or the TD) that were ruled off in ¼ inch intervals. Applicant gradually pulled on these strips using both hands until the label broke. The maximum percentage increase in length among the ruled intervals was recorded.
As part of the research for U.S. Pat. No. 5,836,618, and thereafter, Applicant tested a number of facestock materials that were found suitable for use in fabricating laser-printable, ink jet and thermal transfer-printable cryogenic container labels according to the teaching of this invention. These facestock materials include polypropylene, polyethylene, polyester and various blended facestock materials that even include thermoplastics blended with mineral materials, e.g., Teslin® (a porous composite facestock material containing high density polyethylene combined with silica manufactured by PPG Industries, Pittsburgh, Pa.). Another blended facestock material formerly known as Kimdura® and now known as Yupo® synthetic paper (a combination of polypropylene and calcium carbonate manufactured by the Yupo Corporation of America, Chesapeake, Va.) is also available for such purposes.
More recently, improved PVC labels have been developed that are machine-printable and, in fact, laser-printable. In addition to adding a suitable topcoat to the vinyl so that it retains laser toner, other technical obstacles have been overcome to permit laser printing. For example, laser printer fuser rollers are heated to about 350–400° F. Since PVC begins to melt at only about 160° F., the label must travel quickly and smoothly over the fuser rollers to prevent the PVC from melting (the fuser rollers fuse the laser toner image to the topcoat of the label). Accordingly, the topcoat must be free of any stickiness when it is heated to the temperature of the fuser rollers so that the label stock will not hesitate and melt. Only certain topcoatings are compatible with such high temperatures, and these topcoatings will help dissipate the heat at the surface of the label, while also bonding the laser toner. In addition, a heavier (typically 60–85 pound) paper release liner is often used to support the adhesive-coated (and somewhat softened) label stock. This provides a frictional outer surface that helps feed the label smoothly through the transport rollers without slippage.
Until Applicant conducted research for the present invention using multiple industrial sources of PVC facestocks, vinyl labels were considered unsuitable for use as printable cryogenic container labels. First, a previously developed vinyl cryo-label (3M 7604 FP) could only be manually marked, and had limited utility as a cryo-label due to peeling (see above) particularly as the label aged. In addition, a laser-printable vinyl label material that was recently fabricated for Applicant showed substantial peeling on cryogenic containers when exposed to liquid nitrogen (see Example 1 below). In fact, the importance of using a soft (e.g., highly plasticized) and highly ductile vinyl facestock containing substantially immobilized plasticizer was not appreciated prior to the present invention. Furthermore, the asymmetric stretching properties of certain oriented, e.g., biaxially oriented, vinyl facestocks, and the importance of selecting the facestock orientation in the die-cutting, and placement of such facestocks on cryogenic containers in order to prevent the labels from peeling when exposed to cryogenic temperatures was not appreciated prior to the present invention.